The present invention relates to a brushless motor controller and a method for controlling a brushless motor that are suitable for stabilizing rotational drive, in particular, during rotational driving in a low-speed rotation state immediately after activation.
In a brushless motor, the rotational position of a rotor is detected by a rotation sensor, and the energizing timing of a stator coil is set based on the detected rotational position to control rotation produced by the motor. Japanese Laid-Open Patent Publication No. 2002-315381 and Japanese Patent No. 3420317 describe examples of such a brushless motor.
During rotational driving of a motor, when the drive current increases as the rotational speed of the motor increases, the energizing timing is delayed due to influence of an armature reaction. Therefore, in order to offset the delay, a so-called angle advancing control may be employed. The angle advancing control sets a rotation sensor at an advancing angle side beforehand to advance the energizing timing, or controls the motor at an advancing angle energizing timing that is advanced from a normal energizing timing and set beforehand as described in Japanese Laid-Open Patent Publication No. 2002-315381.
In the angle advancing control, the energizing timing is over-advanced when the rotational speed of a motor is low. For this reason, the advancing angle energizing timing is delayed toward the normal energizing timing. At this time, a count value corresponding to the rotational speed is set by a delay counter, and the energizing timing is delayed based on the count value of the delay counter.
However, during a period immediately after activation in which the speed of the motor is extremely low, it is desirable that the energizing be performed at the normal energizing timing. Thus, the count value, which increases as the rotational speed of the motor decreases, becomes an extremely large value during the low-speed period. However, the count value of the delay counter is finite. Thus, the count value overflows during the period immediately after activation in which the speed of the motor is extremely low. As a result, the energizing timing is not correctly delayed, and the energizing timing remains deviated from the desirable timing. During the period immediately after activation of the motor in which the speed is extremely low, this lowers the motor efficiency and increases noise and vibration.
In Japanese Patent No. 3420317, angle advancing control which uses the advancing angle energizing timing is not performed, and the optimum energizing timing for the present state is calculated from pulse edges of detection signals output from a rotation sensor to control a motor. Thus, the problems described above do not occur during the period immediately after activation in which the speed of the motor is extremely low. However, complicated computations must be performed for every one of the rotational speed ranges. This results in the need for a CPU that performs such complicated computations with a controller and increases the cost of the controller.
In the above described angle advancing control, the rotational drive is stabilized by setting the advancing angle energizing timing based on the rotational speed of the motor. However, further improvement of the stabilization of the rotational drive is desired, and methods for more appropriately setting the advancing angle energizing timing have been contemplated.
It is an objective of the present invention to provide a brushless motor controller and a method for controlling a brushless motor that stabilize rotation with a simple control.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.